Considerable interest exists in the future use of carbon-graphite fibers due to their light weight and high strength. Presently, about 30 pounds of graphite are being utilized per plane in manufacture of several existing aircraft and future projected use is 1000 lbs/aircraft. Due to the need to reduce weight of automobiles to increase fuel efficiency, use in cars is also expected to increase dramatically over the next decade. The projected annual use of graphite fibers by 1990 is as follows:
TABLE 1 ______________________________________ Industry Pounds ______________________________________ Aerospace 10.sup.6 Automobile 10.sup.9 Consumer 10.sup.6 ______________________________________
Commercial carbon fiber is usually sold as a stranded material or as a woven cloth, having from 100 to 10,000 discrete thin fibers per strand. These fibers are prepared by heating a precursor such as rayon, pitch or polyacrylonitrile fiber to carbonize the fibers followed by a high temperature (2000.degree.-3000.degree. C.) graphitization treatment in absence of oxygen during which it is believed that the carbon atoms rearrange into a hexagonal structure. The industry has developed fine strand multifilament products as the result of difficulties in manufacturing large diameter fiber of sufficiently high molulus. It will be noted that an extremely small fiber diameter is now the industry standard, and is not predicted to change very much in the immediate future:
Carbon Fiber Diameter--6.5 to 13 microns PA1 Modulus--50 million psi PA1 Fall Rate--about 2 cm/sec. PA1 Resistivity--1000 ohms/cm. PA1 Burnout--0.5 to 1.0 watt/cm. PA1 Contact Voltage Drop--2 to 5 volts
Recently a significant hazard has been recognized that could prevent the widespread use of graphite fibers. The fine fibers are conductive and are not oxidized nor vaporized at the temperatures experienced during a typical fire. During a fire the epoxy resin binder is consumed at 400.degree.-600.degree. C. Fine graphite fibers and fragments are expelled from the composite, are entrained in the air and form aerosols. The aerosols can travel significant distances, invade or settle in unprotected electrical or electronic equipment and cause shorting, equipment failure, power failure and blackouts. Automobile fires are quite a common event and aircraft fires occur frequently. Such an event could cause disastrous consequences at or near airport, industrial or residential areas.
Since the surface temperature of combustion (fast oxidation) of graphite is in the vicinity of 1300.degree. C., fast oxidation of graphite is hardly reached by the simple combustion of a composite panel which occurs at typical surface temperature of 400.degree.-500.degree. C. Also, even if the requisite temperatures are reached, the rates of combustion (oxidation) are too low compared to the same rates for the resin. This has the practical implication that the resin burns away fast leaving behind the graphite fibers that do not combust in the absence of the supporting flame. The fiber diameter of 8.mu. presents a 2500 cm.sup.2 surface area per gram of mass. This is very large and leads to rapid heat loss and is conducive to early extinction even if the combustion is initiated.
An additional property of the carbon fiber is the "red heat" behavior. It should be emphasized that in a shorting situation a single carbon fiber is most difficult to burn or consume. Rather the literature suggests that the carbon fiber becomes a glowing filament and does not pyrolyze or burn at least to about 2300.degree. K. And even above that temperature adequate air circulation is required to consume the fiber fully. A minimum of 16 grams of oxygen are needed to consume 12 grams of carbon, and hence in a closed area such as in the chassis of an electronic system, lack of air circulation and sufficiently high voltages may cause the fibers to develop a "red heat" condition and ignite adjacent flammable plastics and the like.
In order to permit such widespread use of graphite composites, the recognized electrical hazards must be overcome economically without sacrificing or compromising the proven good features (strength, weight and cost). This aim should preferably be achieved so that the fiber and composite are compatible with state of the art processing and equipment. Furthermore, modification of the fiber by coating or treatment must provide a good bond to the fiber and to the resin matrix.
There have been many proposals to prevent release of electrically conducting graphite fragments from composites in a fire situation. Some approaches have been to gasify the fibers by oxidation or hydrogeneration, clump or retain the fibers so that they do not become airborne, insulate the fibers so no hazards are caused even if the fibers are airborne, resin modifications, alternate fibers, secondary fiber inclusions or larger fibers which would be too heavy for aerosol formation.
Most of these approaches would require several years of investigation at considerable cost, would necessitate requalification of the composite estimated to require several years and would result in substantial modification of properties of the composite and processing techniques for fabricating composites.